1. Field of the Invention
The invention concerns propulsion systems of a spacecraft spin stabilized by rotation about its spin axis, such as a satellite orbiting the earth.
The invention concerns a method of firing the thrusters of a spacecraft propulsion system in order to increase the orbital revolution velocity of the satellite while it is rotating about its spin axis.
2. Description of the Prior Art
A propulsion system may include eight control thrusters for maneuvering the spacecraft by applying a reaction force to it in an appropriate plane in order to adjust its orbital position, its trajectory or its attitude.
Four thrusters are conventionally disposed separately at the periphery of the spacecraft, usually on its circumference, in a plane perpendicular to the spin axis of the spacecraft in order to correct its spin rate and its orbital velocity. The four thrusters are arranged in a known "rectangular" configuration with each thruster at one corner of a rectangle to form two pairs of radially opposed thrusters.
In a known propulsion system arrangement as shown in FIG. 1, the orientations of the thrusters of a pair of thrusters are offset relative to a radial orientation and the thrusters are fired simultaneously for a fraction of the period of revolution of the spacecraft over a number of revolutions in order to increase the orbital velocity of the spacecraft. The thrust axes 40 of the thrusters 30 of a pair of thrusters are so arranged that, during said fraction of the revolution period of the spacecraft 10, the thrust axes of the thrusters of a pair of thrusters are parallel to each other and to the direction D of the maneuver along the orbital velocity vector. The thrusters of a pair of thrusters are activated simultaneously to perform the maneuver, each thruster when activated individually generating an angular moment which modifies the spin rate of the spacecraft.
This propulsion system arrangement has a number of drawbacks and in particular it is found that the jet of propellant gases forming a plume at the thruster nozzles tends to contaminate the experimental equipment and in particular the optical surface of observation devices disposed at the periphery of the spacecraft. Also, the high temperatures of the gas jets have a considerable effect on the performance of the onboard instruments and on the operation of articulated booms carrying instruments and attached to the periphery of the spacecraft. Finally, the rectangular configuration of the thrusters is costly and complex in that it requires a separate propellant feed for each thruster.
It is therefore necessary to optimize the positions of the thrusters and the sequence in which they are fired so as to minimize the harmful effects of the gas jets on the onboard equipment without compromising the performance of the propulsion system in terms of its propellant consumption and its complexity.
An object of the invention is to remedy the drawbacks described above of prior art propulsion systems and in particular to provide a method of firing thrusters in a propulsion system of a spin stabilized spacecraft which minimizes the effect of thruster plume on experimental instruments attached to the exterior of the spacecraft while ensuring reduced propellant consumption during maneuvering of the spacecraft.